The resolution of a two-dimensional digital imaging device is constrained to a given pixel size. In a sensing device, such as a CCD sensor, pixels within the pixel matrix have a fixed size and spacing for detection of an image at a given resolution. Similarly, in a light modulation device, such as a reflective LCD spatial light modulator, pixel size and the spacing of pixels within a two-dimensional array is fixed, constraining the available resolution for forming an image.
Originally developed for forming images in display devices, spatial light modulators are increasingly being used in digital printing applications as well. In printing apparatus, spatial light modulators provide significant advantages in cost and performance over earlier digital imaging technologies, both for line printing systems such as the printer depicted in U.S. Pat. No. 5,521,748, and for area printing systems such as the system described in U.S. Pat. No. 5,652,661.
Two-dimensional area spatial light modulators, such as those using a digital micromirror device (DMD) from Texas Instruments, Dallas, Tex., or using a liquid crystal device (LCD) can be used to modulate an incoming optical beam for imaging at a given resolution. An area spatial light modulator can be considered essentially as a two-dimensional array of light-valve elements, each element corresponding to an image pixel. Each array element is separately addressable and digitally controlled to modulate light by transmitting (or by blocking transmission of) incident light from a light source, typically by affecting the polarization state of the light.
There are two basic types of area spatial light modulators in current use. The first type developed was the transmissive spatial light modulator, which, as its name implies, operates by selective transmission of an optical beam through individual array elements. The second type, a later development, is a reflective spatial light modulator. As its name implies, the reflective spatial light modulator, operates by selective reflection of an optical beam through individual array elements. A suitable example of an LCD reflective area spatial light modulator relevant to this application utilizes an integrated CMOS backplane, allowing a small footprint and improved uniformity characteristics.
Conventionally, LCD area spatial light modulators have been developed and employed for digital projection systems for image display, such as is disclosed in U.S. Pat. No. 5,325,137 and in miniaturized image display apparatus suitable for mounting within a helmet or supported by eyeglasses, as is disclosed in U.S. Pat. No. 5,808,800. LCD projector and display designs in use typically employ one or more area spatial light modulators, such as using one for each of the primary colors, as is disclosed in U.S. Pat. No. 5,743,610.
Spatial light modulators have also been employed in printing apparatus for photosensitive media. Examples of printing apparatus using digital micromirror devices (DMDs), include that disclosed in U.S. Pat. No. 5,461,411. Photographic printers using the more readily available LCD technology are described in U.S. Pat. Nos. 5,652,661; 5,701,185; and 5,745,156, for example.
It is instructive to note that imaging requirements for projector and display use (as is typified in U.S. Pat. Nos. 5,325,137; 5,808,800; and 5,743,610) differ significantly from imaging requirements for printing by photoprocessing apparatus. Projectors are optimized to provide maximum luminous flux to a screen, with secondary emphasis placed on characteristics important in printing, such as contrast and resolution. Optical systems for projector and display applications are designed for the response of the human eye, which, when viewing a display, is relatively insensitive to image artifacts and aberrations and to image non-uniformity, since the displayed image is continually refreshed and is viewed from a distance. However, when viewing printed output from a high-resolution printing system, the human eye is not nearly as “forgiving” to artifacts and aberrations and to non-uniformity, since irregularities in optical response are more readily visible and objectionable on printed output. For this reason, there can be considerable complexity in optical systems for providing a uniform exposure energy for printing. Even more significant are differences in resolution requirements. Adapted for the human eye, projection and display systems are optimized for viewing at relatively low resolutions such as 72 dpi or less, for example. Photographic printing apparatus, on the other hand, must achieve much higher resolution, particularly with apparatus designed for micrographics applications, which can be expected to provide 8,000 dpi for some systems.
Referring to FIG. 1a, there is shown, in simplified form, the basic arrangement of an exemplary prior art imaging apparatus 10 configured as a color printer with separate red, green, and blue (RGB) color channels. There are similar components for modulating each color, represented in FIG. 1a with appended color designators when necessary: r for red, g for green, and b for blue color. A photosensitive medium 32, fed from a reel 34 onto the image plane shown as a surface 36 has characteristic cyan, magenta, and yellow response corresponding to the modulated R, G, B colored light. For the red color light modulation path, a light source 20r provides red light. Uniformizing optics 22r perform basic functions that collect light and provide uniform light for modulation. A polarization beamsplitter 24r directs unmodulated light of a given polarity to a spatial light modulator 30r. The uniformized light from light source 20r is modulated by spatial light modulator 30r, is transmitted through polarization beamsplitter 24r, and is combined at a color combiner, dichroic x-cube 26, with modulated light from corresponding components in the green light path (20g, 22g, 24g, 30g) and blue light path (20b, 22b, 24b, 30b). The modulated color image is then directed by a lens 38 for printing at surface 36. As indicated for the green color channel, the image-forming surface of each spatial light modulator 30 is positionally located at a fixed imaging plane P with respect to the imaging optics.
It must be observed that the arrangement of FIG. 1a represents a limited number of the possible embodiments for imaging apparatus 10 using area spatial light modulators 30. For example, simpler systems can be built using a single spatial light modulator 30 that is shared or multiplexed between two or three light paths, such as that shown in FIG. 1b. In this configuration, a light source 20 provides an illumination beam of red, green, and blue light in a sequence, by means of a filter wheel 28 driven by a motor 18, as is well known in the imaging art. Other methods for directing, as an illumination beam, one color at a time include using separate LEDs having the appropriate color, for example. Uniformizing optics 22 homogenize the illumination beam and provide a uniform field to a polarizing device, such as a polarization beamsplitter 24. Light of suitable polarity for modulation is then directed to a spatial light modulator 30, which modulates the illumination beam with image data that corresponds to the color of the illumination beam provided. For this method, the sequencing of image data corresponds to the sequencing of color in the illumination beam. The modulated color image is then directed by lens 38 for printing at surface 36. Again, the image-forming surface of spatial light modulator 30 is positionally located at an imaging plane P with respect to lens 38 and other imaging optics.
A number of modifications is possible for the configurations of FIGS. 1a and 1b, using techniques well known in the imaging arts. For example, one or more transmissive LCDs could be used instead of the reflective LCDs shown as spatial light modulators 30, 30r, 30g, and 30b, with a suitable rearrangement of support components in each color path.
Referring to FIG. 2, there is shown the arrangement of an ideal imaged pixel array 130 that would be provided by spatial light modulator 30. Pixel array 130 comprises individual pixels 72 arranged in a two-dimensional matrix having evenly spaced rows and columns as shown. A pixel-to-pixel distance D is a factor of the inherent spatial light modulator resolution, and is measured from the center of one pixel 72 to the center of an adjacent pixel 72. As a coarse approximation of the range of displacement distances, a pixel-to-pixel distance D for a typical LCD area spatial light modulator is typically from 10 to 12 microns.
Dithering is one method used for improving the imaging characteristics of pixel array 130. Referring to FIG. 3, there is shown a conventional dithering pattern that has been proposed for compensating for low fill factor of pixels 72 or for increasing pixel resolution. Dither movement of spatial light modulator 30 (FIGS. 1a and 1b) or of some other component in the optics path for modulated light effectively shifts pixels 72 from an original imaging position 78a to a second imaging position 78b, then to a third imaging position 78c, and then to a fourth imaging position 78d. This repeated pattern minimizes the space between pixels to improve pixel fill factor, reducing “pixelization” effects, and increases apparent resolution, as is shown in the dithered pixel array 130 representation of FIG. 4. The image data provided to the spatial light modulator is preferably changed with each shift operation, to effectively provide increased resolution. Conventionally, displacement needed for dithering is a fraction of a pixel; however, multiple-pixel dithering is also possible. Commonly-assigned U.S. Pat. Nos. 6,552,740 and 6,547,032 disclose various dithering approaches for imaging apparatus employing area spatial light modulators.
The same type of technique, using controlled incremental motion as shown in FIG. 3, can be used for increasing the effective resolution of an image sensor, such as a CCD array, for example. In the imaging arts, the term “dithering” has been used primarily in a printing context. However, for the purposes of this disclosure, this term is used with broader application, to describe the type of pixel displacement described with reference to FIG. 3 for both image-sensing and image-forming devices. For any of these devices, as was described with reference to FIGS. 1a and 1b, dithering provides movement that is substantially within the fixed plane P of the surface of spatial light modulator 30. As shown in the conventional coordinate axis representation of FIG. 3, dithering typically provides displacement in the directions of mutually orthogonal x and y axes, which lie within plane P in the context of FIGS. 1a and 1b. There is, however, no appreciable displacement in the direction of the z axis, that is, in the direction of incident light, chiefly in order to maintain correct focus. Moreover, any dithering mechanism must also constrain any rotational displacement about the z axis (referred to as θz).
Various mechanisms for providing controlled dithering motion and thereby increasing effective image resolution have been proposed, with application to various fields, including the following:                Commonly-assigned U.S. Pat. No. 5,400,070 discloses an imaging sensing apparatus using a tiltable refraction plate for redirecting incoming light to a solid state image sensor, such as a charge-coupled device (CCD), where a motor-actuated cam is used to provide suitable tilting action;        U.S. Pat. No. 5,786,901 discloses an image shifting device using piezoelectric actuators to control the tilt of a refraction plate in the optical path of an image sensor;        U.S. Pat. No. 5,557,327 discloses a mechanism for pixel shifting within an image sensing apparatus using motors cooperating with spring constraints to tilt a refractive element to one or more positions;        U.S. Pat. No. 4,449,213 discloses an optical reading apparatus providing X-Y displacement using electromagnetic actuation to shift the position of an objective lens;        U.S. Pat. No. 4,581,649 discloses a system for improved image detection using dithering motion caused by solenoid actuation;        U.S. Pat. No. 4,607,287 discloses vibration of an image sensor using piezoelectric actuators to achieve higher image resolution;        Commonly-assigned U.S. Pat. No. 5,063,450 discloses a dithering motion in a camera for prevention of aliasing, wherein an image sensor is mounted onto a piezoelectric actuator;        U.S. Pat. No. 4,633,317 discloses an electro-optical detector system using a dithered image offset controlled using electromagnetic actuators driving a reflective member; and        U.S. Patent Application Publication No. 2003/0063838 discloses a beam steering apparatus using piezoelectric actuators for cross-connect switching of optical signals.        
As the above listing shows, mechanisms employed for providing the displacement needed for dithering have included electromagnetic and piezoelectric actuators. These devices can achieve precision movement over various ranges, depending on the device type. However, while each of the above-mentioned approaches has merit for a particular application, given a specific displacement distance, prior art approaches have not provided a low-cost, precision dithering mechanism that can, at the same time, be adapted for a range of different displacement distances and meets rigid criteria for compactness, robustness, and adjustability. Moreover, there are advantages to solutions that do not interpose added optical components, such as glass plates, which can be sensitive to dirt and dust and may introduce unwanted optical effects.
Thus, it can be seen that there is a need for an apparatus and method for achieving controlled dither motion of an imaging device in orthogonal directions, within a fixed plane.